This is the most annoying problem in the world! When I'm doing my lossless clips with Virtualdub and AviSynth, suddenly the vdub screen becomes grey and at the bottom of the window there's written this:

Or you could update to a newer build of FFMS2. That issue was fixed way back in June (with r691). And while there is something on FFMS2's bug tracker that claims it happens with 'raw rgb input', the ticket doesn't say what revision of FFMS2 is being used, and the sample file that supposedly shows it is offline. So I'm not apt to trust it as a valid ticket in the first place.